1. Technical Field
The present invention relates to a press forming die assembly including a draw bead for controlling the amount of a blank material that flows into a draw-forming portion in press forming. Specifically, the present invention relates to improvements in the shape of a draw bead used in press forming of a composite blank material that includes sheet materials having different sheet thicknesses and rigidities.
2. Background Art
Panels, such as door panels for vehicles, are produced by press forming a composite blank material including plural sheet materials that have different sheet thicknesses and rigidities and are integrally connected by butt-welding.
A die assembly used in press forming includes a draw forming portion that has a draw forming concave portion, a draw forming convex portion, and a forming surface corresponding to a product surface of a panel. The concave portion is provided at the center of a die, and the convex portion is provided at an end portion of a punch. In the press forming die assembly, a composite blank material is held by a peripheral portion of the die and a blank holder, and the convex portion of the punch is moved to the concave portion of the die. As a result, the composite blank material flows into the draw forming portion, whereby a panel having a convex product surface, for example, is formed. In this case, a draw bead is provided on the holding surfaces of the peripheral portion of the die and the blank holder in a direction approximately parallel to the outer peripheral shape of the draw forming portion. The flow amount of the composite blank material is controlled by the draw bead in press forming.
When a blank material having a thin sheet portion and a thick sheet portion is used as a composite blank material, a step portion is provided to an upper die of a press forming die assembly. The step portion corresponds to the difference of the sheet thickness between the thin sheet portion and the thick sheet potion. The step portion is welded with a material that is harder than the base material of the die, and a sharp edge is formed thereon by a finish processing. In press forming, since there is a difference of inflow shear amount due to the difference in sheet thicknesses at the connected portion of the thin sheet portion and the thick sheet portion, the thick sheet portion may overlap with the thin sheet portion. Such overlap is prevented by the above step portion.
In the case of using the step portion, forming failure such as buckling wrinkling easily occurs due to the difference in sheet thicknesses of the thick sheet portion and the thin sheet portion. Accordingly, adjustment operation for a die assembly requires time in trial forming, and forming failure may be unavoidable in a product having a complicated shape.
Moreover, the following problems may occur in commercial production. FIGS. 7A and 7B show an example of a composite blank material 210 having a panel 220 that is formed by a conventional press forming die assembly. FIG. 7A shows a top view, and FIG. 7B shows a cross sectional drawing taken along line 7B-7B. As shown in FIG. 7A, when the composite blank material 210 moves to a draw forming portion, rotation R of the material of a thick sheet portion 211 (movement of a connecting line 213 of the thick sheet portion 211 and the thin sheet portion 212 toward the thin sheet portion 212) occurs. Then, a crack H is generated at the connecting line 213. In this case, a sharp edge of the thick sheet portion 211 passes through the die assembly, whereby there is wear of the die assembly, and particles are generated and may be trapped at a product surface 221 of the panel 220.
As shown in FIGS. 7A and 7B, buckling wrinkling I is generated at the thin sheet portion 212 by the rotation R of the material of the thick sheet portion 211. When the composite blank material 210 is used for an inner panel of a vehicle door, an end surface of an outer panel is connected to an inner panel by a hemming process. In this case, the end surface of the outer panel may be degraded by the buckling wrinkling of the inner panel (for example, rough portions may be produced or adhesion failure may occur at the end surface of the outer panel).
Moreover, because of the rotation R of the material of the thick sheet portion 211, strain is concentrated in the thin sheet portion 212, and the flow of the material into the draw forming portion is decreased. Therefore, as shown in FIG. 7A, a crack J may be generated at a ridge line portion 222 in a raised wall of the product surface 221. FIG. 7A shows a reference numeral 214, which indicates a bead portion formed by a draw bead of a press forming die assembly, and a reference numeral 223, which indicates a flange portion of the thick sheet portion 211. In addition, FIG. 7A shows a reference numeral F, which indicates a flow direction of a material in draw forming.
As described above, in the conventional press forming die assembly, forming failure may occur in the vicinity of the connected portion of the thick sheet portion and the thin sheet portion. Accordingly, adjustment operation of a die assembly is time consuming in trial forming, and the quality of products may be degraded in commercial production.
In order to solve the above problems, several techniques have been disclosed to improve the shape of the above draw bead for controlling the flow amount of a material. For example, Japanese Unexamined Patent Application Publication No. 2001-259751 discloses a technique for generating a large flow resistance. In this technique, a draw bead is formed in a trapezoidal shape. In a boundary area including a connected portion of a thin sheet portion and a thick sheet portion of a composite blank material, the height, the width, and the curvature radius of corners of the trapezoidal shape are greater than those in an area other than the boundary area. However, in this technique, the material of the thick sheet portion may be rotated by movement of the composite blank material to a draw forming portion. Moreover, according to the increase in the flowing resistance, the sheet thickness of a ridge line portion in a raised wall on a product surface of a panel is decreased. Therefore, the above problems cannot be solved. Specifically, when a corner of a draw forming portion is close to the connected portion of the thick sheet portion and the thin sheet portion, the above problems become serious. The above problems occur in press forming of a composite blank material, which has different rigidities, including a portion with high rigidity and a portion with low rigidity.